In metal product mining processes such as gold and silver mining processes, valuable metal is typically liberated from a metal product-bearing ore by leaching of the ore with a sodium cyanide solution at pH of about 9 or above. This process also causes the leaching of mercury into the solution as well where it interferes with the subsequent purification and processing of the metal product.
Mercury contamination in the Gold-Cyanide Process (GCP) is a serious health and environmental problem. Following the leaching with sodium cyanide at high pH, an ore leachate (also referred to as a cyanidation solution) containing dissolved metal-cyano species is collected and contacted with activated carbon in order to extract the gold from the solution. In Carbon Adsorption and Desorption processes, portions of the mercury-cyano complexes present in the ore leachate also readily adhere to the activated carbon and are thus carried forward as a contaminant into subsequent processing steps of electrowinning and carbon regeneration. In Merille-Crowe processing (Zinc precipitation) of gold, the mercury can be extracted from the precipitant by retorting under vacuum and high temperature, however this mercury will then need to be disposed of and risks exposure to workers. During these steps the contaminant mercury is often emitted into the air as a vapor. This poses a severe health hazard to plant workers and the local environment. Additional concerns relate to the safety of workers when handling the mercury contaminated materials and solutions.
The amount of mercury contamination present in an ore leachate varies depending on the particular ore. In some cases, mercury contamination is negligible. In other cases, as much as 50 ppm or even up to 100 ppm mercury is present in a cyanidation solution.
It would be highly desirable to remove some or all of the mercury present in a mercury-containing ore leachate while also substantially avoiding the removal of the metal product (e.g. gold or silver). By providing a method of preferential separation, mercury is prevented from contacting the activated carbon, resolving the worker health issues and environmental issues described above. Further by providing such a separation, the yield of metal product in the solution is maximized in subsequent processing steps.
Currently, there are a paucity of methods that are useful to preferentially reduce or remove mercury from ore leachates, such as gold and silver ore leachates and in particular gold ore leachates. Due to chemical similarities in particular between gold and mercury and additionally the known tendency of both gold and silver to form amalgams with mercury, substantial difficulties remain in recovering the valuable metal product. In the Merrill-Crowe process, evaporation of mercury from zinc prepitated gold remains an effective but extremely undesirable method of choice in many ore refining operations since this still leads to risk of mercury exposure and the mercury must be handled and disposed at great effort and expense.